THE Ulysse Nardin wristwatch in an ad in the most recent Sunday edition of The New York Times is unlike 22 of the 24 watches featured in that issue’s ads, but chances are that didn’t register with most readers. For horologists, however, whose scrutiny of watches tends toward the Talmudic, it’s a lapel grabber: all the other watches — from brands like Rolex, TAG Heuer and Gucci — are set at 10:10, but Ulysse Nardin’s watch is set at 8:19. (The only other exception is an Oris wristwatch, one of four featured in an ad by the retailer Tourneau, which is set at 8:03.)

In a recent check of the 100 top-selling men’s dress watches on Amazon.com, which included models from 20 brands, all but three watches were set to 10:10. To be watch-shopping online and first notice that every model arrayed on the screen is set to an identical time can feel like crossing over into the Twilight Zone.

But the explanation turns out to be a simple matter of aesthetics.

Because brand names generally are centered on the upper half of a watch, hands positioned at 10 and 2 “frame the brand and logo,” said Andrew Block, executive vice president at Tourneau, the watch retailer, which has 51 stores worldwide. “It’s almost like an unwritten rule that everyone understands to photograph a watch a 10:10.”

In previous eras, the more popular time in ads was 8:20, which shared the attributes of being symmetrical and not overshadowing logos, but hands pointing down struck some as, well, a downer.

“It has the aesthetic of the smiley face to be 10 past 10, so we try whenever possible to opt for that,” Susanne Hurni, head of Ulysse Nardin’s advertising and marketing, said from the company headquarters in Le Locle, Switzerland. She says the company occasionally makes exceptions, as it does for models now advertised in publications including The Wall Street Journal and The New York Times, when watches have secondary dials and windows — for the day of the week, calendar day and year — that would be obscured by the hands at 10 and 2.

Klaus Peter Mager, a spokesman for Swatch, said his 25-year-old company, based in Biel, Switzerland, has always photographed watches primarily at 10:10, because “they’re smiling instead of a sad man’s face.” About 30 percent of the more than 400 models Swatch introduces yearly are photographed set at different times so that the hands don’t obscure functions, he said.

But Timex never deviates, even if that means the hands block features, said Adam Gurian, president of Timex, which is based in Middlebury, Conn. The company has an official time, 10:09:36, at which every watch — even digital models — is photographed for marketing purposes. Having the second hand at 36 tends to accommodate secondary language — like “Indiglo,” its dial-lighting technology — which appears centered at the bottom of watches.

To preserve batteries, the company ships many watches turned off at 10:09:36, which lends synchronicity to Timex displays in store windows.

At Rolex, watches are always photographed at 10:10:31, and for models that list the day of the week and calendar day, it is always Monday the 28th. A survey of hundreds of vintage wristwatch print ads posted online — in galleries at Adclassix.com, at the watch enthusiast site TimeZone.com, and on eBay — indicates that 10:10 was not always the norm. Watches in the 1920s and 1930s were almost exclusively set at 8:20.

The Hamilton Watch Company was among the first to clock in at 10:10; that time is favored in ads dating at least as far back as 1926. Rolex began consistently setting watches in ads at 10:10 in the early 1940s. Timex appears to have begun the transition in 1953, when its Ben Hogan model showed 8:20 while the Marlin model was set to 10:10.

Linda Kaplan Thaler, chief executive of the Kaplan Thaler Group, a New York advertising agency, learned about the 10:10 rule when her firm worked on a campaign for Rolex several years ago, and was drawn to the notion that it was like a smile.

“In advertising we would never expect someone to look at a watch and say, ‘The watch is smiling,’ but it’s just a feeling you get,” said Ms. Kaplan Thaler, co-author, with Robin Koval, of “The Power of Nice,” which features a big smile on its cover. The watch theme, she added, is typical of “subconscious cues that are used in print ads.”

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Watchmakers are, naturally, fretting over how to sell watches to a generation that is in the habit of consulting their phones for the time, so it is perhaps fitting that the most-hyped phone has its own time-related intrigue. Many bloggers have wondered why the time on the iPhone in commercials, with few exceptions, reads 9:42 a.m., even when the capability being highlighted on the phone — like watching the “Pirates of Penzance” and being compelled to order calamari from a seafood restaurant — might seem atypical behavior over the day’s first cup of coffee.

The most popular theory is that it was 9:42 a.m. Pacific Time when Steve Jobs introduced the iPhone at a MacWorld conference in 2007, a fact confirmed by live blogs from the conference, but two press officers from Apple did not return calls seeking an explanation.

Watch companies, meanwhile, have the unenviable task of creating ads that will be dissected by aficionados, who are by nature obsessed with precision. Ms. Hurni of Ulysse Nardin learned this painfully more than a decade ago, when preparing a watch with day, month and year features for a shoot. Ms. Hurni always sets the calendar date as much as a year ahead, ensuring that the ad will not look dated, but after she set the watch in an ad several months ahead to Sunday, March 19, 1996, some customers sent calendars to the company’s Swiss headquarters to underscore that March 19 would actually fall on a Tuesday.

That makes sense to Michael Sandler, the general manager of TimeZone.com, who several years ago noticed that an out-of-focus model in the background of a Patek Phillipe ad was wearing her watch upside down, a slip-up he doubts was recognized by nonhorologists.

“Watch geeks are interesting people,” Mr. Sandler said. “They’ll pick up on weird stuff like that from an ad.”